1. Field of the Invention
The present invention relates to slurry distribution systems, and more particularly to slurry distribution systems used with polishing machines in the manufacture of semiconductor wafers.
2. Description of Related Art
In the manufacture of integrated circuits, planarization of semiconductor wafers is becoming increasingly important as the number of layers used to form integrated circuits increases. For instance, metallization layers that provide interconnects between various devices may result in nonuniform surfaces. The surface nonuniformities may interfere with the optical resolution of subsequent photolithographic steps, leading to difficulty with printing high resolution patterns. The surface nonuniformities may also interfere with step coverage of subsequently deposited metal layers and possibly cause open or short circuits.
Various techniques have been developed to planarize the top surface of a semiconductor wafer. One such approach involves polishing the wafer using a polishing slurry that includes abrasive particles mixed in a suspension agent. With this approach, the wafer is mounted on a wafer holder, a polishing pad coated with the slurry is mounted on a platen, the pad and the wafer are rotated such that the wafer provides a planetary motion with respect to the pad, and the polishing pad is pressed against an exposed surface of the wafer with a hydrodynamic layer of the slurry therebetween. The polishing erodes the wafer surface, and the process continues until the wafer is largely flattened.
In chemical-mechanical polishing, the slurry includes a chemical that assists the abrasive particles in removing wafer material. Chemical-mechanical polishing has become a popular wafer planarization technique. For instance, chemical-mechanical polishing is becoming a preferred method of planarizing tungsten interconnects, vias and contacts, and with proper process parameters has shown significantly improved process windows and defect levels over standard tungsten dry etching.
Typically, the slurry is mixed in bulk by adding the abrasive particles and any additives, oxidizers, etchants and/or de-ionized water to the suspension agent. The slurry is then transported in containers to a slurry distribution system and deposited in a storage tank. The slurry distribution system pumps the slurry from the storage tank through delivery lines to a polishing machine as polishing occurs. It is desirable to provide a uniform distribution of the abrasive particles in the slurry in order to provide evenly polished surfaces. As a result, mixing devices are used to mix the slurry in the storage tanks in order to prevent large amounts of the abrasive particles from settling to the bottom of the storage tanks.
Polishing machines are operated with periodic intervals between polishing the semiconductor wafers. For instance, after a given wafer is polished, it is usually necessary to rinse the wafer with de-ionized water, remove it from the polishing machine, and secure another wafer to the polishing machine before polishing commences again. During intervals between polishing, when the slurry is not dispensed, it can remain stationary in the delivery lines, causing large amounts of the abrasive particles to settle to the bottom of the delivery lines. Thereafter, when polishing begins, and the slurry that had been stationary in the delivery lines is dispensed to the polishing machine, the uneven distribution of abrasive particles can lead to unevenly polished surfaces. Furthermore, the settled abrasive particles can clog the delivery lines.
This problem can be addressed by continuously dispensing the slurry to the polishing machine, or by flushing the stationary slurry out of the delivery lines without using it to assist with polishing. However, these approaches waste a considerable amount of slurry, which can be a significant cost burden in manufacturing.
Accordingly, a need exists for a slurry distribution system that efficiently provides agitated slurry to polishing machines.